The assignee of the present invention presently markets an enteral fluid infusion device under the trademark "KANGAROO." This system includes a fluid infusion device and a disposable fluid delivery set, which includes a fluid inlet tube, a drip chamber which is arranged to be mounted in a recess on the fluid infusion device, a mounting member, also arranged to be mounted in a recess on the fluid infusion device, an outlet tube connected to the mounting member and a stretchable pump tube or silicone tube which connects the drip chamber to the mounting member and engages a motor driven infusion control member or rotor on the fluid infusion device to form a peristaltic infusion pump.
As disclosed in U.S. Pat. No. 4,913,703 granted to Pasqualucci et al. and assigned to the assignee of the present invention, the mounting member preferably includes a magnetic member which is sensed by a sensor such as a magnetic sensor on the infusion device to enable operation of the fluid infusion device only when the pump tube is operatively mounted or aligned about the rotor of the fluid infusion device. The disclosure of U.S. Pat. No. 4,913,703 is incorporated herein by reference as if fully set forth herein below.
The preferred form of the motor units used in the "KANGAROO" fluid infusion devices is disclosed in U.S. Pat. No. 4,884,013 granted to Jackson et al. and assigned to the assignee of the present invention. The disclosure of this patent is incorporated herein as if fully set forth below. As disclosed in the Jackson et al. patent, a means for detecting the completion of an operating cycle of the pump operating means and also operating the motor unit until the completion of the operating cycle is detected. Operation of the motor unit is repeated at variable time intervals or cycle times which are selected in accordance with the desired rate of fluid delivery. More specifically, the motor unit operates under the control of a microcomputer which is provided with a first control program. A programmable interval timer is provided for operating and initiating the microcomputer. Additionally, a clock operating at approximately 4 Mhz, provides clock pulses to the present system. The various controls of the motor unit are proved as input signals to the microcomputer. Additional inputs to the microcomputer are provided by magnetic sensors which sense the mounting member (as set forth more fully in U.S. Pat. No. 4,913,703 discussed above) and which sense a plurality of magnets on the rotor as the magnets rotate past another magnetic field sensor during each operating cycle.
One common feature of many of the commercially available infusion devices is that the flow of fl id through the fluid delivery set is not directly measured. Instead, the flow of fluid through the fluid delivery set is estimated based on a predetermined or average volume of fluid located between a pair of members which compress or pinch the tubing and push the fluid through the fluid delivery set. These members may be oriented linearly such as in a linear peristaltic infusion device or as part of a rotor in a rotary peristaltic infusion device. Because the flow of fluid through the fluid delivery set is estimated, there are a number of factors which may affect the actual flow of fluid to the patient. Two of the most correctable causes of inaccurate fluid delivery to a patient relate to variances in the inner diameter of the pump tube and variances in the length of the pump tube used on the fluid delivery set. The incidence of inaccurate fluid delivery with either of these causes may be significantly reduced by carefully monitoring the critical dimensions of the fluid delivery sets.
The development of a fluid infusion device which is designed for ambulatory use and in a conventional setting such as a hospital, requires that another cause of inaccurate fluid delivery be addressed. This further cause of inaccurate fluid delivery relates to the height of the fluid reservoir above the infusion control member on the fluid infusion device (referred to herein as the head height). When a fluid infusion device is used on an IV pole, the fluid reservoir is typically hung two or more feet above the infusion control member. The gravitational flow of fluid from the fluid reservoir to the infusion control member causes a first fluid pressure within the fluid delivery set. This fluid pressure has a particularly pronounced affect on the stretchable pump tube portion of the fluid delivery set. In most currently available fluid infusion devices, the fluid delivery set is particularly designed for use in this situation and the operation of the infusion control device is programmed based on this amount fluid pressure inside the fluid delivery set.
During ambulatory use, the fluid reservoir is typically positioned adjacent to the fluid infusion device such that the outlet of the fluid reservoir is positioned at or near the height of the infusion control member. This causes a second fluid pressure in the fluid delivery set which is significantly lower than the first fluid pressure created when the fluid infusion device is used on an IV pole. Because of the lower second fluid pressure when the fluid infusion device is used as an ambulatory device, less fluid will be trapped between the members in the infusion control member. This occurs because the second fluid pressure will cause less stretching of the pump tube than the first fluid pressure. Therefore, significantly less fluid may be delivered to the patient during the same number of revolutions of the rotor on a rotary peristaltic infusion device or the same number of movements of the members on a linear peristaltic infusion device. Conversely, if the fluid reservoir is somehow hung a greater than normal distance above the infusion control member, a higher fluid pressure will be present in the fluid delivery set and excess fluid will be delivered to the patient.
Currently, a need remains for a device which will compensate for the changes in fluid pressure within the fluid delivery set due to the relative position of the fluid reservoir with respect to the infusion control member on the fluid infusion device.